Add paddle.sparse and three Sparse API

[zkh2016]

PR types

New features

PR changes

APIs

Describe

1. 添加路径：paddle.sparse，paddle.sparse.functional，paddle.sparse.layer
2. 添加API：paddle.sparse.sparse_coo_tensor（用于创建COO格式的Sparse Tensor ）和 paddle.sparse.sparse_csr_tensor（用于创建CSR格式的Sparse Tensor ）
3. 添加API：paddle.sparse.ReLU

使用例子：

import paddle
from paddle.fluid.framework import _test_eager_guard
# 当前需要在eager模式下使用sparse API，后面会统一去掉
with _test_eager_guard():
   indices = [[0, 1],[0, 1]]
   values = [-1.0, 2.0]
   sparse_x = paddle.sparse.sparse_coo_tensor(indices, values, [2, 2])
   sparse_relu = paddle.sparse.ReLU()
   out = sparse_relu(sparse_x)
   # print(out.non_zero_elements())
   # [0.0, 2.0]

英文文档：

[paddle-bot-old]

你的PR提交成功，感谢你对开源项目的贡献!
请关注后续CI自动化测试结果，详情请参考Paddle-CI手册。
Your PR has been submitted. Thanks for your contribution!
Please wait for the result of CI firstly. See Paddle CI Manual for details.